This invention relates to a method of manufacturing the target of an image pickup tube, and more particularly to a method of manufacturing a photoconductive film having a hetero-junction.
In the manufacturing of the target of a pickup tube, it has recently been proposed to deposit an N-type light transmitting conductive film consisting of indium oxide (In.sub.2 O.sub.3) or tin oxide (SnO.sub. 2) on a light transmitting substrate of glass or quartz and then depositing a P-type amorphous photoconductive film consisting essentially of metallic selenium incorporated with tellurium (Te) and arsenic or the like for the purpose of preventing crystallization of the N-type conductive film in which the N-type light transmitting film and the P-type amorphous photoconductive film form a hetero-junction, thereby improving the spectral sensitivity characteristic, response speed characteristic, dark current characteristic, resolution characteristic, ease of production and other various characteristics, and such proposal has succeeded to some extent thus far.
However, when the target of a pickup tube constructed as described above is left standstill or operated at an environmental temperature above 40.degree.C, due to the difference in the coefficient of thermal expansion of N-type and P-type films, stress is created along the interface between the two films. Thus when the pickup tube is left exposed to a high temperature or used in a high temperature atmosphere the dark current increases greatly with the result that the quality of the picture will be degraded greatly.
To overcome this difficulty I have proposed an improved target of a pickup tube wherein an N-type photoconductive film composed of compounds of the materials of groups II and VI of the periodic table such as cadmium selenide (CdSe), cadmium sulfide (CdS); zinc sulfide (ZnS) and zinc selenide (ZnSe), is interposed between the N-type light transmitting conductive film and the P-type amorphous photoconductive film.
According to this improved construction, the tendency of crystallization of P-type amorphous photoconductive film is more efficiently reduced by forming the N-type light transmitting film to have a desired thickness, and at the same time the problem of creating stress along the interface between the N-type light transmitting conductive film and the P-type amorphous photoconductive film when the ambient temperature rises beyond 40.degree.C can also be solved. Further, the dark current and the number of twinkling defects are greatly decreased when compared with the prior art construction.
However, it was found that it is not always possible to obtain an excellent target by merely increasing the thickness of the N-type photoconductive film and that when the condition of manufacturing the film departs from the optional a satisfactory result can never be attained.